I have been playing the piano since I _(be) 6 years old.填was,been,还是其他的什么?

问题描述:

I have been playing the piano since I _(be) 6 years old.
填was,been,还是其他的什么?

was

I have been playing the piano since I was 6 years old.
现在完成时since后跟一般过去式,固定搭配,没有为什么